Wake up. America’s military isn’t invincible.

U.S. Marines stand guard during the change of command ceremony at Task Force Southwest military field in Shorab military camp of Helmand province, Afghanistan. The Pentagon is developing plans to withdraw up to half of the 14,000 American troops serving in Afghanistan. (Massoud Hossaini/AP)

The most uncovered story in Washington these days is the loss of U.S. military power — a lesson particularly important in light of recent events: the resignation of Defense Secretary Jim Mattis; President Trump’s rash decision to withdraw U.S. troops from Syria; North Korea’s announcement that it will keep nuclear weapons after all; and alleged massive computer hacking by Chinese nationals.

After the Cold War, Americans assumed that no other country could match the United States in its military might and technological leadership. The reality, long known in the military, is that defense-modernization programs in Russia and China, as well as advances in Iran and North Korea, threaten to leapfrog U.S. capabilities.

The military plays two essential roles in defending U.S. international goals. The first is to deter aggression that would jeopardize American interests, because potential adversaries believe their chances of prevailing are slim to none. The second is to fight — and to win — wars to protect those same interests. In both cases, the United States’ military is on a downward trajectory.

Skeptics, please read the recent report of the congressionally created National Defense Strategy Commission, a group of civilian experts and retired military officers. Here are a few quotes from the report:

— “If the United States had to fight Russia in a Baltic contingency or China in a war over Taiwan . . . Americans could face a decisive military defeat.”

— “America has reached the point of a full-blown national security crisis.”

My fellow Post columnist Max Boot has done a great favor by publicizing the report. Writes Boot: “Air superiority, which the United States has taken for granted since World War II, is no longer assured. And, without control of the skies, U.S. ships and soldiers would be [highly] vulnerable.”

The slippage in our military power has at least three causes, only two of which we can influence.

The first is other countries’ decisions to beef up their militaries; we can’t change that. The second is the shifting nature of warfare, with the rise of cyberwarfare and other new technologies (communications satellites and the like). We can do better here by addressing the third cause: unwise cuts in defense spending.

Look at the numbers: From fiscal 2010 to 2015, defense spending fell 26 percent, from $794 billion (in inflation-corrected 2018 dollars) to $586 billion, according to the NDSC report. Excluding the costs of Iraq and Afghanistan, the decline is 12 percent, from $612 billion to $541 billion over the same years.

The time has long passed since the Pentagon was the driving force behind the federal budget. In 1960, defense was 52 percent of federal outlays and 9 percent of overall economic activity (gross domestic product). In 2017, the comparable figures were 15 percent of outlays and 3 percent of GDP.

In truth, military spending is in a quiet competition with the American welfare state — Social Security, Medicare, Medicaid, food stamps and the like — which now represents roughly 70 percent of budget outlays.

The Pentagon is losing badly. Welfare programs have vast constituencies of voters. Defense has fewer. Politicians straddle the conflict. They vote for welfare while insisting that the U.S. military is still the world’s most powerful. This rationalizes inaction on defense but conveniently forgets that the military’s margin of superiority has dramatically shriveled.

Meanwhile, potential adversaries are arming themselves. The U.S.-China Economic and Security Review Commission, another congressionally created watchdog group, reports that China has recently introduced a new stealth fighter (the J-20) and its “growing ballistic and cruise missile inventory . . . can target U.S. bases and surface ships, including aircraft carriers.”

Similarly, Russia last week claimed to have successfully tested a ship-borne hypersonic missile that travels at roughly eight times the speed of sound. The United States is said to have no defense against it.

Does all this seem familiar? Well, yes. It’s hard to miss the parallels with the period before World War II, when England, France and the United States allowed Adolf Hitler to rearm Germany, altering the global balance of power. The delusional complacency recalls John F. Kennedy’s book, “Why England Slept.”

This is not a call for war. It is a call for stopping many self-inflicted wounds. We need to stop underfunding the military, especially on research and cyberwarfare, even if that means less welfare. We need to keep our commitments — Trump’s abrupt withdrawal from Syria devalues our word. And we need to repair our alliances.

War is changing, and we need to change with it. Otherwise, we may drift into a large war impossible to win. Surely we don’t need a book called “Why America Slept.”